This invention is generally directed to an overcoated photoresponsive imaging member; and more specifically, the present invention is directed to an improved overcoated photoresponsive member which contains a trapping layer and an overcoating layer, and the use of such devices in electrostatographic imaging systems, wherein only one charging step is needed for causing the formation of images. The overcoating layer selected for the imaging member of the present invention contains a composition that is capable of transferring or donating electrons to positive corona ions on the members surface, thus allowing electrons to move upward, and positive charges to move downward towards the substrate as shown hereinafter.
Electrophotography, and in particular xerography, which involves the formation and development of images on an imaging surface, such as a photoconductive material, is well known. Numerous different types of photoreceptors have been developed for this purpose, including materials wherein the charge carrier generation and charge carrier transport function are accomplished by discrete contiguous layers. Additionally, there are known photoreceptors which include an overcoating layer of an electrically insulating polymeric material, and in conjunction with this overcoated type photoreceptor there have been proposed a number of imaging methods. Thus, for example, there has been described in U.S. Pat. No. 4,254,199 the utilization of an overcoated photoreceptor employing a double charging process that is, the photoreceptor device is subjected to charges of a positive polarity, and subsequently such devices are subjected to charges of a negative polarity. The photoreceptor device employed can be comprised of a substrate, overcoated with a hole injecting layer, which in turn is overcoated with a charge transport layer, followed by an overcoating of a charge generating layer, and finally a top overcoating layer of an insulating organic resin. This device is fully described in U.S. Pat. No. 4,251,612.
More specifically, the imaging process as described in U.S. Pat. No. '199 involves the charging of the photoresponsive device a first time with electrostatic charges of a first polarity, followed by charging a second time with electrostatic charges of polarity opposite to the first polarity, in order to substantially neutralize the charges residing on the electrically insulating surface of the device, followed by exposing the device to an imagewise pattern of activating electromagnetic radiation, whereby an electrostatic latent image is formed. The electrostatic latent image may then be developed to form a visible image, which can then be transferred to a receiving member. Subsequently, the imaging member may be reused to form additional reproductions after the erasure and cleaning steps have been accomplished.
In another known process employing overcoated photoreceptor devices, there is selected a non-ambipolar photoconductor wherein charge carriers are injected from the electrode substrate into the photoconductor surface. In such a system, in order to obtain high quality images, the injecting electrode must satisfy the requirement that it injects carriers efficiently, and uniformly into the photoconductor material.
U.S. Pat. No. 3,041,167 teaches an electrostatographic imaging method which employs an overcoated imaging member comprising a conductive substrate, a photoconductive insulating layer, and an overcoating layer of an electrically insulating polymeric material. This member is utilized in an electrostatographic copying method by, for example, initially charging the member with an electrostatic charge of a first polarity, and imagewise exposing to form an electrostatic latent image which can then be developed to form a visible image. The visible image is transferred to a receiver member, and the surface of the imaging member is cleaned to complete the imaging cycle. Prior to each succeeding cycle, the imaging member can be charged with an electrostatic charge of a second polarity, opposite in polarity to the first polarity. Sufficient additional charges of the second polarity are applied so as to create across the member a net electrical field of the second polarity. Simultaneously, mobile charges of the first polarity are created in the photoconductive layer such as by applying an electrical potential to the conductive substrate. The imaging potential, which is developed to form the visible image is present across the photoconductive layer, and the overcoating layer.
Various other imaging methods are known such as those described by Mark, in an article appearing in "Photographic Science and Engineering," Volume 18, No. 3, pages 254-261, May/June, 1974. The process referred to by Mark as the Katsuragawa and Canon processes can basically be divided into four steps. The first is step involves charging a device containing an insulating overcoating. This is normally accomplished by exposing the device to a DC corona of a polarity opposite to that of the majority charge carrier. When applying a positive charge to the surface of the insulating layer, as in the situation where an n-type photoconductor is employed, a negative charge is induced in the conductive substrate, injected into the photoconductor, and transported to and trapped at the insulating layer-photoconductive layer interface resulting in an initial potential being solely across the insulating layer. The charged plate is then exposed to a light and shadow pattern while simultaneously applying to its surface an electronic field of either alternating current (Canon) or direct current of polarity opposite that of the initial electrostatic charge (Katsuragawa). The plate is then uniformly exposed to activating radiation to produce a developable image with potential across the insulating overcoating, and to simultaneously reduce the potential across the photoconductive layer to zero. In other processes described in the Mark article, i.e., the Hall and Butterfield processes, the polarity of the initial voltage is the same sign as the majority charge carrier and reverse polarity is encountered during erase.
While the devices described in the prior art function properly and adequately under most circumstances, there continues to be a need for improved overcoated photoreceptor devices, particularly devices which contain an electron donating overcoating material, and a trapping layer, and which devices can be utilized in imaging systems employing one charging step rather than the customary two charging steps. Further, the art of electrostatography and more specifically xerography, continues to advance, and more stringent demands need to be met by the copying apparatus in order to increase performance standards, obtain higher quality images; and further there is a need for improved protective photoreceptor overcoatings. Also there is a need for improved overcoated photoresponsive devices where during the imaging process the manner in which, and the type of charges that are transported and retained at various levels of the photoreceptor device, can be controlled. Also, there continues to be a need for improved overcoated photoresponsive devices wherein an injecting layer is not needed, and wherein a single charging step can be utilized.